The present invention relates to tone receivers for communication systems, and more particularly, to multifrequency tone receivers for pulse-code-modulation (PCM) communication systems.
In prior art communication systems, tone receivers have typically been of the analog type since both speech signals and supervisory tone signals have been transmitted over communication channels in analog form. More recently, communication systems have adopted digital transmission techniques, such as PCM, because digital techniques provide very high quality signal transmission which is not degraded over long distances. Thus, since both speech signals and supervisory tone signals are digitally encoded in such communication systems, a digital tone receiver is preferable over a conventional analog tone receiver.
For detecting tones in a sampled digital signal, a digital tone receiver may utilize Fourier spectrum techniques, such as that described in the article entitled, "Digital MF Receiver Technique" by Jeffrey P. Mills, published in the GTE Automatic Electric Journal, Volume 15, No. 7, May 1977, at pages 317-325, or may utilize digital filter techniques, such as that described in the article entitled, "Special Purpose Hardware for Digital Filtering," by Stanley L. Freeny, published in the Proceedings of the IEEE, Vol 63, No. 4, April 1975, at pp. 633-648. The Fourier spectrum techniques described in the foregoing article require a large number of floating-point multiplication operations, which necessitate costly, high speed multipliers that have relatively high power dissipation ratings. Furthermore, the Fourier spectrum type tone receivers cannot readily provide filter operations having a number of cascaded filters which may be varied in both number and order under program control.
In the case of digital filter techniques, tone receivers utilizing digital filters are much slower than the Fourier spectrum type of tone receivers since a number of multiplication operations are required to implement each filter. For example, three multiplication operations are required for each second-order digital filter. Because multiplication operations are time consuming, tone receivers utilizing digital filters may only accommodate digital signals having relatively low sampling frequencies. Furthermore, such tone receivers would require a different digital filter therein to detect each different tone signal that may occur in a multifrequency tone signal. Thus, prior art tone receivers either require a large number of high speed multipliers as in the Fourier spectrum type of tone receiver, or require a large number of digital filters, each requiring a large number of relatively slow multiplication operations as in the digital filter type of tone receiver.